1. Field of the Invention
The present invention relates to a hinge board useful as a board for an electronic device such as a folding portable telephone, and a method for producing the hinge board.
2. Related Background Art
In recent years, an increase in a memory capacity of an electronic device has advanced wiring miniaturization, high-density wiring and high-speed electronic signals. Accordingly, a demand level of mechanical properties for a wiring board has been set higher. Moreover, for a hinge board having a hinge bending part, which is used for a folding portable telephone, miniaturization of wiring and the like are similarly demanded. Thus, the hinge board has been required to have a multilayer structure and to have its thickness reduced. However, a conventional multilayer hinge board has a problem that, when a hinge bending part is repeatedly bent, there occurs disconnection in a conductor circuit. Thus, one having sufficient hinge flexibility in a hinge part of the folding portable telephone has not necessarily been obtained.
Therefore, for example, Japanese Patent Application Laid-Open Gazette No. Hei 7-312469 (JP 7-312469 A) has disclosed a structure of a bending part of a multilayer flexible circuit board which is formed in the following manner. Specifically, in the multilayer flexible circuit board, at least one layer of a flexible insulating base material is bonded, by use of an adhesive layer, between a plurality of wiring conductors in which a required wiring pattern is formed by use of a conductive foil. Moreover, in the multilayer flexible circuit board, a non-bonded part is provided so that the flexible insulating base material in the bending part of the multilayer flexible circuit board is not bonded to the wiring conductor or another flexible insulating base material at least on one side. In the specification, it is described that the multilayer flexible circuit board is formed by laminating adhesive-free one-side copper-clad plates.
However, in the multilayer flexible circuit board described in Document 1, an adhesive layer is required to laminate each of the one-side copper-clad plates. Thus, in the case where the circuit board is formed to have a multilayer structure, a thickness thereof is increased and sufficient high-density wiring cannot be achieved. In the above point, the multilayer flexible circuit board described above is not satisfactory yet.